α,ω-Hydroxy telechelic poly(propylene oxide) (PPO) is widely used industrially as a midsegment in polyurethane synthesis. These polymers have been produced from racemic propylene oxide using chain shuttling agents and double-metal cyanide catalysts that produce atactic polymers. Unlike atactic PPO, isotactic PPO is semicrystalline with a melting temperature of approximately 67° C. Currently there is no practical route to α,ω-hydroxy telechelic isotactic PPO using racemic propylene oxide.
Polymers with reactive end-groups are useful materials as they can be readily integrated into more complicated macromolecular assemblies in a well-defined manner. For example, low molecular weight polymers with multiple terminal hydroxy groups (α,ω-hydroxy telechelic polymers, also known as “polyols”) react with multi-functional isocyanates to form polyurethanes, a class of polymers with an estimated production of 17 billion pounds in 2010 (˜5% of worldwide polymer production) that are used to form a large variety of products such as soft and rigid foams, adhesives, and elastomers. The functionality (i.e., the number of hydroxyl groups) of the polyol used affects the polyurethane product. Diols produce linear polyurethanes when reacted with diisocyanates, while polyols with functionality greater than two produce cross-linked polyurethanes. Atactic poly(propylene oxide) (aPPO) polyols are commonly used due to their low cost and the desirable properties they impart to the final polyurethane.
Numerous catalysts capable of the stereorandom polymerization of propylene oxide (PO) are known, but only alkali hydroxide and double-metal cyanide (DMC) catalysts are commonly used industrially. While aPPO diols can be prepared from hydroxyl initiators, the main route to aPPO polyols uses protic chain shuttling agents (CSAs) that give polyols with the same functionality as the parent CSA. CSAs are reagents that allow for the production of multiple polymer chains per catalyst center and control of polymer molecular weight by varying the monomer to CSA ratio. They function by reacting with a propagating polymer chain at a catalytic center to produce a new propagating polymer chain and a latent polymer chain, which can later behave as a CSA and reinitiate propagation, such as the addition of alcohols to the polymerization of PO with (tetraphenylporphinato)aluminum chloride catalysts led to lower molecular weight polymers with ether and hydroxyl end-groups, an “immortal polymerization.”
Similar to polypropylene, atactic PPO is amorphous with a Tg of approximately −70° C., while isotactic PPO (iPPO) is a semicrystalline solid with a Tm of approximately 67° C. Unlike polypropylene, however, the production of iPPO is more challenging than that of aPPO, severely limiting investigations into its applications. There are three routes to iPPO. In route A, enantiopure PO is polymerized in a regioregular fashion. Unfortunately, the high cost of enantiopure PO renders this approach uneconomical. In route B, a chiral, enantioselective catalyst polymerizes rac-PO to give enantiopure iPPO and unreacted, enantiopure PO of the opposite stereoconfiguration. While our group has prepared enantiopure bimetallic (salen)Co(III) catalysts for this transformation, most reported epoxide polymerization catalysts are achiral or suffer from low enantioselectivities. Route C is similar to route B, except with a chiral, racemic catalyst that is used to form isotactic polymer chains from both enantiomers of the epoxide. This route is especially appealing for the large-scale production of iPPO, as all of the PO can be polymerized in one step and the stereoregularity of the polymer is not degraded as conversion to polymer increases. Although numerous heterogenous catalysts are known to produce a mixture of iPPO and aPPO chains, there are only a few catalysts capable of solely producing iPPO chains, including (salph)Co(III) complexes and homogenous racemic bimetallic (salen)Co(III) complexes.
Only a few reports of α,ω-hydroxy telecheclic iPPO and their use as polyurethane midsegments have been disclosed, mostly from chain-scission of high molecular weight iPPO produced from partially-isoselective heterogeneous catalysts. Generally, mixtures of iPPO and aPPO were reported to be useful polyurethane midsegments for flexible foams and elastomers, while using solely iPPO imparted improved properties to rigid polyurethane foams. An early study on polyurethane peel strengths and rigidity found that a 3000 Da 40% isotactic PPO diol produced a polyurethane with higher peel strength and rigidity than its atactic counterpart. A tin phosphate condensate catalyst has been reported for the production of telechelic iPPO, but the resulting polymers have levels of mm-triad content ([mm]) below 60% and broad molecular weight distributions (2.4-4.6). Furthermore, preparation of the catalyst requires fractionation, and isolation of the isotactic polymer product necessitates removal of atactic polymer and other side-products. Tokunaga et al. reported in a patent that semicrystalline diol iPPOs can be formed by polymerizing PO with the (salph)Co(III) catalyst and related compounds in the presence of acetic acid and then hydrolyzing the resulting acetate end-groups with KOH. The molecular weights of these polymers are controlled by the amounts of catalyst and acetic acid used, though it is not clear if the acetic acid controls the molecular weight by acting as a CSA or through other means, as the ratio of PO to acetic acid (˜6) is too low for the molecular weights produced (˜2600 Da) if all of the acetic acid functions as a CSA. The solid-state nature of the (salph)Co(III) catalyst is also not addressed.